Why do some flowers trigger sneezing fits in people, while others do not?

Visitors to this year's Chelsea Flower Show will be able to find out the answer in an exhibition by the Royal College of Pathologists.

The display includes plants which most people know can cause a reaction, such as wheat.

But it also feature less familiar allergens, such as primulas and birch trees.

Exhibitors stress none of these plants will be flowering - so they should not trigger people's allergies.

Another area is planted with low allergenic plants that can minimise symptoms in susceptible individuals.

These are plants such as double geraniums, which are sterile and do not generate pollen and therefore do not trigger allergies.

Patients will be able to look at microscope slides showing human tissue samples - such as samples from the lungs of asthmatics.

Viruses

Dr Tim Wreghitt, who organised the display for the college, said: "We'll be advising people on which plants to put in their garden which will lower their chance of allergies - the ones to choose are F1 hybrids, which don't reproduce.

We'll be educating people about how the body reacts to an allergen

Dr Tim Wreghitt, Royal College of Pathologists

"And we'll be educating people about how the body reacts to an allergen, and what the chemical mechanisms are."

He added that part of the aim was to tell people a little more about what pathologists actually did.

Pathology covers a range of disciplines including virology - the study of viruses and viral diseases - and histopathology, which is the study of changes in tissues.

Dr Wreghitt told BBC News Online: "Most people think that we just chop up dead bodies.

"But the vast majority of our job is actually helping the living by working on tissue samples from living people.

"Pathologists contribute to 70% of diagnoses made.

"But often we are backroom boys, and we're not dealing directly with the public."

Seaweed

The Royal College of Pathologists has won a Royal Horticultural Society Silver-gilt Medal at Chelsea for the second year running.

Last year's display demonstrated how plants could harm humans and how those diseases were treated.

It also showed how plants were used in pathological diagnosis, including how seaweed is used to make the substance in which bacteria like meningitis are grown from patients' samples

Around 157,000 people are set to visit this year's show, which will feature 600 exhibitors.

The Chelsea Flower Show 2004 runs from Tuesday 25 to Friday 28 May 2004 in the grounds of the Royal Hospital, Chelsea, London.